AC ground mystery

[karista]

Using a circuit tester for my AC outlets, it showed a open ground at the GFI outlet, but it showed good correct ground settings at all other AC outlets.
These outlets are all controlled by the GFI which shows a open ground. Any ideas how there can be proper ground connections at all outlets except the GFI outlet that controls all the others? I would assume therefore, there is no GFI protection on all the other AC circuits!